1. Field of the Invention
This invention relates to the imaging systems and, more particularly, to the recording of x-ray images.
2. Description of the Prior Art
In the conventional x-ray analysis of objects or tissues, an x-ray image is formed by causing x-rays to traverse the object or tissue by which the x-rays are differentially absorbed. A photographic film is then exposed to the x-ray image, either directly or through the intermediary of a fluorescent screen activated by the x-ray image. A recent system, in particular, makes use of an ultraviolet phosphor screen activated by the x-ray image, in combination with a photographic film sensitive to the ultraviolet light. However, photographic films are expensive and have limited enhancement capabilities.
One prior art arrangement for the electrostatic recording of x-ray images utilizes a cathode and an anode spaced with a gas-filled gap therebetween, with a voltage being applied across the cathode and the anode. The cathode is made of, or coated with, a heavy metal and a plastic sheet is adjacent to the anode. A pattern of x-rays is directed on the heavy metal cathode which emits photoelectrons in response thereto. The photoelectrons are accelerated across the gas-filled gap and strike the gas molecules, forming ion-electron pairs and producing an "avalanche effect" in the gas. As a result thereof, a pattern of electric charges corresponding to the pattern of x-rays is deposited on the plastic sheet adjacent to the anode. After exposure, the cathode is removed and the electrostatic charge on the plastic layer is developed by means of conventional xerographic techniques.
The principal disadvantage of such an arrangement is that the heavy metal directly excited by the x-rays emits a small number of photoelectrons per each incident x-ray photon. As a matter of fact, the electron escape length in all solids is much shorter than the x-ray absorption length. Hence, only those x-rays which are absorbed within a few microns of the cathode surface have the chance of producing a photoelectron. As a result thereof, the efficiency of such a heavy metal cathode is very poor, i.e., the number of photoelectrons emitted by the heavy metal cathode for each x-ray photon impinging thereupon, is low. Accordingly, the quality of the recording is poor.
Another prior art arrangement makes use of an electrode comprising a layer of a fluorescent material adjacent to a layer of photoelectric film. The x-ray image is converted into a visible light image by means of the fluorescent material; the visible light image is converted into an electron image by means of the photoelectric film. The electrons are then accelerated by a series of focusing elements and strike on a fluorescent screen, where they produce a great-intensity image which may be easily photographed. The electrode, the focusing elements, and the fluorescent screen are enclosed within an evacuated envelope. The principal disadvantages of this arrangement are the need for an evacuated envelope which, for medical applications, must be very bulky and expensive and, moreover, the need for an expensive photographic film. It is to be noted that the photoelectric flim used in the electrode of this prior art arrangement cannot be exposed to air, since it would be destroyed by oxidation. Accordingly, such an electrode cannot be used for all applications in which the electrodes must be exposed to air. In particular, it cannot be used in combination with the electrostatic recording system of x-ray images discussed above, since in that system the cathode must be removed and exposed to air for developing the image.
Another prior art arrangement consists of two planar parallel metal electrodes, with the gap between the electrodes filled with high pressure xenon gas, with an electric field applied across the gap, and with a plastic sheet adjacent to one of the electrodes. X-rays are absorbed in the xenon gas and each x-ray photon produces many electron-ion pairs. The electric field accelerates the electrons (or the ions, according to the field polarity) to the plastic sheet. The charge image on the plastic sheet is subsequently developed with conventional xerographic techniques. The principal disadvantages of this arrangement are the need for a high pressure in the gas, which results in the need for a thick, but x-ray transparent, window and the high cost of xenon gas. After each exposure the xenon gas must be recovered and repurified; accordingly, a complicated and expensive purification system is required.